1. Field of the Invention
The present invention relates to a process for thermally bonding plastic parts which are not amenable to adhesive bonding. More particularly, the invention relates to a process for thermally bonding plastic parts, such as two polyolefinic substrates, e.g., tubings, one of which is highly crosslinked, to produce a bond of narrow width over a predetermined circumscribed area which has high strength against peeling or shearing.
2. Description of the Prior Art
Heretofore the bonding of plastic tubings, such as the bonding of a crosslinked plastic tubing to a non-crosslinked plastic tubing, has been accomplished utilizing conventional thermal bonding or adhesive bonding processes.
The adhesive bonding process utilizes an epoxy or structured acrylic which may cause a reduced strength against peeling attributable to loosening of the seal or to a loss of adhesiveness of the epoxy or acrylic due to environmental attack on, or aging of, the adhesive.
Conventional thermal bonding processes require the application of heat to areas of a tubing greater than one inch in length. Such large areas over which the heat is applied commonly contain "hot spots" and the heating of such a large area can result in an uneven shrinking or splitting of one tubing relative to another tubing which are being bonded together during cooldown of the tubings after bonding.
Various processes for bonding plastics are disclosed in the following U.S. Patents:
______________________________________ U.S. Pat. No. PATENTEE ______________________________________ 3,467,561 Waride 3,832,253 DiPalma, et al. 4,003,382 Dyke 4,251,305 Becker, et al. 4,263,236 Briggs, et al. Re. 30,817 Loyd, et al. ______________________________________
The Waride U.S. Pat. No. 3,467,561 discloses a method of insulating a joint formed in a conductor or electrical cable by applying a first winding of heat fusible synthetic plastic tape over the joint and a second winding of tetrafluoroethylene tape and then heating the tape windings for unification.
The DiPalma et al. U.S. Pat. No. 3,832,253 discloses a method of making an inflatable balloon catheter wherein a number of heat shrinkable sleeves and adhesive are utilized to attach a catheter tip to a catheter shaft.
The Dyke U.S. Pat. No. 4,033,382 discloses a method of forming a retention catheter wherein a thermoplastic polyurethane body is bonded to a thermosetting polyurethane balloon. The thermosetting polyurethane balloon sleeve is molded with integral bands of a thermoplastic polyurethane polymer spaced at either end of the sleeve and is sealed to the body by fusing the thermoplastic polyurethane polymer bands to the body by application of heat or solvent.
The Becker, et al. U.S. Pat. No. 4,251,305 discloses a method and apparatus for the radiant heat sealing of a balloon onto a catheter, the method employing the use of tinted shrink tubing sections in areas to be bonded and the apparatus being automatic to provide heat to the areas along the catheter to be bonded for short periods of time while feeding the catheter automatically into the machine for the bonding.
The Briggs, et al. U.S. Pat. No. 4,263,236 discloses a method for forming a catheter with an inflation cuff. The catheter is formed of a thermoplastic material while the cuff is formed of an elastomeric material which is not thermoplastic. Once the cuff is placed over the catheter, pressure is directed inwardly around the entire periphery of at least each end of the cuff while the catheter is being heated sufficiently to soften same so as to enable at least the ends of the cuff to be recessed into the material of the catheter by the inwardly directed pressure.
The Loyd, et al. U.S. Reissue Pat. No. Re. 30,817 discloses a method of applying a laminated tubular insulating connector for splicing or terminating electric conductors. The connector comprises an outer layer of a heat shrinkable dielectric material and an inner layer of a relatively soft, self-adhesive, low temperature flowable thermoplastic material. When heated, the outer layer shrinks and the inner layer functions as a sealant and cushioning agent.
The process of the present invention differs from the previously proposed methods by providing a narrow circumferential section of bonding between two tubings where intricate fittings are required, i.e., when special tips of materials different than the material from which a catheter tubing is made are required, and a very intimate contact between the tubings is formed by providing an inward compressive force during heating of the tubings with the resultant bond having increased strength against peeling or shearing as will be described in greater detail hereinafter.